Photoelectric effect

Photoelectric effect, phenomenon in which electrically charged particles are released from or within a material when it absorbs electromagnetic radiation. The effect is often defined as the ejection of electrons from a metal plate when light falls on it. In a broader definition, the radiant energy may be infrared, visible, or ultraviolet light, X rays, or gamma rays; the material may be a solid, liquid, or gas; and the released particles may be ions (electrically charged atoms or molecules) as well as electrons. The phenomenon was fundamentally significant in the development of modern physics because of the puzzling questions it raised about the nature of light—particle versus wavelike behaviour—that were finally resolved by Albert Einstein in 1905. The effect remains important for research in areas from materials science to astrophysics, as well as forming the basis for a variety of useful devices.

Discovery and early work

The photoelectric effect was discovered in 1887 by the German physicist Heinrich Rudolf Hertz. In connection with work on radio waves, Hertz observed that, when ultraviolet light shines on two metal electrodes with a voltage applied across them, the light changes the voltage at which sparking takes place. This relation between light and electricity (hence photoelectric) was clarified in 1902 by another German physicist, Philipp Lenard. He demonstrated that electrically charged particles are liberated from a metal surface when it is illuminated and that these particles are identical to electrons, which had been discovered by the British physicist Joseph John Thomson in 1897.

Further research showed that the photoelectric effect represents an interaction between light and matter that cannot be explained by classical physics, which describes light as an electromagnetic wave. One inexplicable observation was that the maximum kinetic energy of the released electrons did not vary with the intensity of the light, as expected according to the wave theory, but was proportional instead to the frequency of the light. What the light intensity did determine was the number of electrons released from the metal (measured as an electric current). Another puzzling observation was that there was virtually no time lag between the arrival of radiation and the emission of electrons.

Consideration of these unexpected behaviours led Albert Einstein to formulate in 1905 a new corpuscular theory of light in which each particle of light, or photon, contains a fixed amount of energy, or quantum, that depends on the light’s frequency. In particular, a photon carries an energy E equal to hf, where f is the frequency of the light and h is the universal constant that the German physicist Max Planck derived in 1900 to explain the wavelength distribution of blackbody radiation—that is, the electromagnetic radiation emitted from a hot body. The relationship may also be written in the equivalent form E = hc/λ, where c is the speed of light and λ is its wavelength, showing that the energy of a photon is inversely proportional to its wavelength.

Einstein assumed that a photon would penetrate the material and transfer its energy to an electron. As the electron moved through the metal at high speed and finally emerged from the material, its kinetic energy would diminish by an amount ϕ called the work function (similar to the electronic work function), which represents the energy required for the electron to escape the metal. By conservation of energy, this reasoning led Einstein to the photoelectric equation Ek = hf − ϕ, where Ek is the maximum kinetic energy of the ejected electron.

Although Einstein’s model described the emission of electrons from an illuminated plate, his photon hypothesis was sufficiently radical that it was not universally accepted until it received further experimental verification. Further corroboration occurred in 1916 when extremely accurate measurements by the American physicist Robert Millikan verified Einstein’s equation and showed with high precision that the value of Einstein’s constant h was the same as Planck’s constant. Einstein was finally awarded the Nobel Prize for Physics in 1921 for explaining the photoelectric effect.

In 1922 the American physicist Arthur Compton measured the change in wavelength of X rays after they interacted with free electrons, and he showed that the change could be calculated by treating X rays as made of photons. Compton received the 1927 Nobel Prize for Physics for this work. In 1931 the British mathematician Ralph Howard Fowler extended the understanding of photoelectric emission by establishing the relationship between photoelectric current and temperature in metals. Further efforts showed that electromagnetic radiation could also emit electrons in insulators, which do not conduct electricity, and in semiconductors, a variety of insulators that conduct electricity only under certain circumstances.

Photoelectric principles

According to quantum mechanics, electrons bound to atoms occur in specific electronic configurations. The highest energy configuration (or energy band) that is normally occupied by electrons for a given material is known as the valence band, and the degree to which it is filled largely determines the material’s electrical conductivity. In a typical conductor (metal), the valence band is about half filled with electrons, which readily move from atom to atom, carrying a current. In a good insulator, such as glass or rubber, the valence band is filled, and these valence electrons have very little mobility. Like insulators, semiconductors generally have their valence bands filled, but, unlike insulators, very little energy is required to excite an electron from the valence band to the next allowed energy band—known as the conduction band, because any electron excited to this higher energy level is relatively free. For example, the “bandgap” for silicon is 1.12 eV (electron volts), and that of gallium arsenide is 1.42 eV. This is in the range of energy carried by photons of infrared and visible light, which can therefore raise electrons in semiconductors to the conduction band. (For comparison, an ordinary flashlight battery imparts 1.5 eV to each electron that passes through it. Much more energetic radiation is required to overcome the bandgap in insulators.) Depending on how the semiconducting material is configured, this radiation may enhance its electrical conductivity by adding to an electric current already induced by an applied voltage (seephotoconductivity), or it may generate a voltage independently of any external voltage sources (seephotovoltaic effect).

Photoconductivity arises from the electrons freed by the light and from a flow of positive charge as well. Electrons raised to the conduction band correspond to missing negative charges in the valence band, called “holes.” Both electrons and holes increase current flow when the semiconductor is illuminated.

In the photovoltaic effect, a voltage is generated when the electrons freed by the incident light are separated from the holes that are generated, producing a difference in electrical potential. This is typically done by using a p-n junction rather than a pure semiconductor. A p-n junction occurs at the juncture between p-type (positive) and n-type (negative) semiconductors. These opposite regions are created by the addition of different impurities to produce excess electrons (n-type) or excess holes (p-type). Illumination frees electrons and holes on opposite sides of the junction to produce a voltage across the junction that can propel current, thereby converting light into electrical power.

Other photoelectric effects are caused by radiation at higher frequencies, such as X rays and gamma rays. These higher-energy photons can even release electrons near the atomic nucleus, where they are tightly bound. When such an inner electron is ejected, a higher-energy outer electron quickly drops down to fill the vacancy. The excess energy results in the emission of one or more additional electrons from the atom, which is called the Auger effect.

Also seen at high photon energies is the Compton effect, which arises when an X-ray or gamma-ray photon collides with an electron. The effect can be analyzed by the same principles that govern the collision between any two bodies, including conservation of momentum. The photon loses energy to the electron, a decrease that corresponds to an increased photon wavelength according to Einstein’s relation E = hc/λ. When the collision is such that the electron and the photon part at right angles to each other, the photon’s wavelength increases by a characteristic amount called the Compton wavelength, 2.43 × 10−12 metre.

Applications

Devices based on the photoelectric effect have several desirable properties, including producing a current that is directly proportional to light intensity and a very fast response time. One basic device is the photoelectric cell, or photodiode. Originally, this was a phototube, a vacuum tube containing a cathode made of a metal with a small work function so that electrons would be easily emitted. The current released by the plate would be gathered by an anode held at a large positive voltage relative to the cathode. Phototubes have been replaced by semiconductor-based photodiodes that can detect light, measure its intensity, control other devices as a function of illumination, and turn light into electrical energy. These devices work at low voltages, comparable to their bandgaps, and they are used in industrial process control, pollution monitoring, light detection within fibre opticstelecommunications networks, solar cells, imaging, and many other applications.

Photoconductive cells are made of semiconductors with bandgaps that correspond to the photon energies to be sensed. For example, photographic exposure meters and automatic switches for street lighting operate in the visible spectrum, so they are typically made of cadmium sulfide. Infrared detectors, such as sensors for night-vision applications, may be made of lead sulfide or mercury cadmium telluride.

Photovoltaic devices typically incorporate a semiconductor p-n junction. For solar cell use, they are usually made of crystalline silicon and convert about 15 percent of the incident light energy into electricity. Solar cells are often used to provide relatively small amounts of power in special environments such as space satellites and remote telephone installations. Development of cheaper materials and higher efficiencies may make solar power economically feasible for large-scale applications.

The photomultiplier tube is a highly sensitive extension of the phototube, first developed in the 1930s, which contains a series of metal plates called dynodes. Light striking the cathode releases electrons. These are attracted to the first dynode, where they release additional electrons that strike the second dynode, and so on. After up to 10 dynode stages, the photocurrent is so enormously amplified that some photomultipliers can virtually detect a single photon. These devices, or solid-state versions of comparable sensitivity, are invaluable in spectroscopy research, where it is often necessary to measure extremely weak light sources. They are also used in scintillation counters, which contain a material that produces flashes of light when struck by X rays or gamma rays, coupled to a photomultiplier that counts the flashes and measures their intensity. These counters support applications such as identifying particular isotopes for nuclear tracer analysis and detecting X rays used in computerized axial tomography (CAT) scans to portray a cross section through the body.

Photodiodes and photomultipliers also contribute to imaging technology. Light amplifiers or image intensifiers, television camera tubes, and image-storage tubes use the fact that the electron emission from each point on a cathode is determined by the number of photons arriving at that point. An optical image falling on one side of a semitransparent cathode is converted into an equivalent “electron current” image on the other side. Then electric and magnetic fields are used to focus the electrons onto a phosphor screen. Each electron striking the phosphor produces a flash of light, causing the release of many more electrons from the corresponding point on a cathode directly opposite the phosphor. The resulting intensified image can be further enhanced by the same process to produce even greater amplification and can be displayed or stored.

At higher photon energies the analysis of electrons emitted by X rays gives information about electronic transitions among energy states in atoms and molecules. It also contributes to the study of certain nuclear processes, and it plays a role in the chemical analysis of materials, since emitted electrons carry a specific energy that is characteristic of the atomic source. The Compton effect is also used to analyze the properties of materials, and in astronomy it is used to analyze gamma rays that come from cosmic sources.